The present invention relates to an X-ray computed tomography apparatus for measuring required projection data by using the X-ray and rotating a scanner around an object, in particular to a method and apparatus for X-ray computed tomography capable of providing reconstructed images with high resolution by shifting the position of the X-ray focal spot of an X-ray source with respect to the scanner.
FIG. 2 shows a typical configuration of the conventional X-ray computed tomography apparatus of the third generation. On a scanner 1, Nb X-ray detector elements d0 through d(Nb-1) are disposed on the opposite side of an object 4 from an X-ray source 2. The X-ray source 2 emits an X-ray 5 in a fan beam form. X-ray intensities transmitted through the object 4 are detected by the X-ray detector elements d0 through d(Nb-1) and measured. Data thus measured are called projection data. The projection data are sent to an image reconstructor 7 via a multiplexer 6.
Centering around a center 3 of rotation, the scanner 1 rotates around the object 4 and emits a fan beam 5 to the object 4. The X-ray detector elements d0 through d(Nb-1) measure projection data derived from all directions. On the basis of all projection data thus acquired, the image reconstructor 7 reconstructs the distribution of X-ray attenuation coefficient on a measured cross section of the object 4 as an image. This reconstructed image is displayed on a screen of a display 8. As a calculation method in the image reconstructor 7, the "filtered back-projection method", for example, is known.
As a method for improving the spatial resolution of reconstructed images in X-ray computed tomography apparatuses, a method of obtaining tomographic images while switching over X-ray focal spot position every projection has been proposed (in JP-A-4-231940 and U.S. Pat. No. 5,173,852). Hereafter, this method is called interlacing method.